The question was defined as 'the one you use the most': HP 48SX. For the shirt pocket: the ultra slim, ultra light (weighs almost nothing!) "M-Abacus", which does 'everything' except programming, yes 'everything' (hypertrig, stats, complex, etc.) An incredible bargain at $7! Although algebraic :-(

You said "...I wish HP people inspire on this inquiry to build up future models...". And I'd like to complete this statement: "... and rebuild older models, like the fascinating 9100A/B!" It's quite easy to build a new calculator from scratch using today's technology, but to build a scientific calc 1968 with no ICs available, a non-volatile core mem and including transcendental functions for the trigonometrics and logarithms - that's real innovation!

Now you know what my favorite HP is... but I'm old-fashioned of course ;-)

Seriously, there are ICs in the HP9100B. Even if you don't count the ROM PCB (multi-layer with inductive coupling between the tracks -- and HP considered it to be an IC at one point)
or the 1810-0021 hybrids in the flip-flop circuits, there are still ICs in the machine.
8 op-amps on the card reader PCB. 2 for each of the 4 tracks (3 dtata tracks, one timing track). One for each of the read preamps, one for each of the comparators.
OK, the card readers is sort-of optional (the machine will run with it removed), and it's not part of the processor logic, but there are still ICs in the 9100B

Whether _all_ the logic is discrete parts depends on how you consider the 1810-0021 RC network. It contains half the resistors and capacitors for a JK flip-flop. 2 of the RC networks, 4 transistors and a few diodes (the semiconductors all being discrete parts) make a flip-flop. 2 of the PCBs inside are just 20 JKs each....
As for the design, I regard it as one of the most beautiful designs _ever_. It's elegantly simple (it's very simple, considering what it does). There's no adder -- just increment and decrement. Oh, and shifts, transfer between registers, and simple things like that.
It's a pity so few people really understand what's going on inside that machine.

Are you seriously asking me to attempt to draw an HP9100 schematic using ascii-art in this forum?
What exactly do you want to know about this machine? I don't know much about the firmware, but I've fixed the hardware side a couple of times and know roughly waht is going on.

Yep, that's a pity ;-) So come on and enlighten us with brilliant wisdom! Tell us where to find detailed information about the mode of operation of the *really* incredible 9100A/B or post it somewhere on the web - and please do not just link to hpmuseum.org; everybody here should know this one...

BTW, look at this URL: http://www.chac.org/engine-ascii/engv2n3.txt. It is a quite interesting interview with Barney Oliver, who is well known for his engagement at SETI and HP. Most interesting is the following text passage:

...
Oliver: Yeah, I think that's right. I know there were six
planes. I'm not sure of the details.

KC: _Don't worry, we have full docs on the 9100. Which, of
course, are several times the size of the machine itself._

Oliver:(laughs) Very good! There were something like 2200 bits
-- yeah, 2208 bits of core memory. About 32,000 bits of ROM.
...

What does "KC" mean with "full docs"? Does anyone know the guys behind chac.org?

Before anyone complains: Sorry for "abusing" this theme ("A calc poll...") for discussing the HP-9100A/Bs mode of operation. I'll post the last passage (the link to the interview) in a new theme in the forum because I think it is possibly worth discussing.

There are two calculators of which I use one almost as much as the other.

The first is the HP-16C, which I use a lot at my job as a software engineer. Haven't found any other that is as convenient to use (I think the method used by the HP-48 series to enter numbers in bases other than 10 is a real pain!)

The calculator I use for everything else is an HP-67. I just love LED RPN calculators.

Do I really have to pick just _one_ calculator? OK, it has to be the HP41. Very expandable, nice hardware, and HPIL.
However, I'd also like to mention : HP48SX (in my pocket all the time, RPL is a nice programming language), HP71B (probably the nicest pocket _computer_ ever, HP97 (clear display, pretty nice keystroke programming language, and a built-in printer) (And the HP67, which is more portable if I don't need the printer), and of course the HP9100 (one of the best designs ever).
But I think I still want to put the HP41 'top of the list'.
I don't use financial calculators (I don't have enough money to need one :-)), so I won't consider those.

Only that I bought a 48SX when it came out, but never bought the 48GX. I bought the 48SX because it did have features that I wanted that weren't on mu 28C (specifically the serial port). The GX does have a lot more
functions, but I didn't really need them, so I didn't buy it. The 49G (which I did buy) has a horrible build quality, but it's also got a lot of nice functions.

May I include the VERY desirable 1.5MB user mem (1M Flash, 512K RAM) and reloadable OS to the list of goodies available in the 49G? In time: will we, poor mortals, have access to the 49 O.S. builder (if this stuff exists)?

Though I've recently been using MathUPro and Coconut on my Palm more often.

HP 16C

Nothing like it for integer math!

HP 41CX

'Nuff said!

Financial:

HP 17BII

Generally more useful than a 12C (IMHO) and has an RPN mode!

HP 10B

It hangs out on my desk at work, for trivial stuff.

Sorry for the extras under each category. It's kinda' like asking me to pick my favorite standard and metric wrench -- I really can't limit myself to just one. If you really just want one from each, use my first answer.

HP-41CX for pure fun, hackability and gadgets. It was fun at any level, from applications right down to machine code.. I guess this would be it if I only had one choice.

But the 20 series, especially the 29C was nice to use. Could be operated with one hand and had a solid "feel". The 27 was an interesting mix of scientific and finance. Neat. I still use the 29C quite a bit, although partly to keep the ni-cads in good shape.

Of course the 15C and 16C for their uniqueness, battery life and shirt pocket size. The only thing I wish the 15C had would perhaps be a clock and time functions.

Never ever charge your nicads in the 29C. Always use the external charger or another machine. The 29C is probably the most prone to die machine HP ever built (followed by the 25C). If the battery contacts are dirty, the pack falls out, or the pack is old the internal voltage rises to chip-lethal levels.

Yes, thank you for the advice. I was fortunate in that
I picked up the little external battery pack holder and
charger when I originally bought my 29C in the seventies.
I didn't realize what a smart buy that was back then, but
I'm glad to have it now.

I have 3 battery packs for it, one of which is one I made
up with some new Ni-Mh cells. I wasn't sure this would
work, but it works just fine. I rotate them through the
machine and external pack holder to charge them. (The
other two have original ni-cads and still work great.)

With all the machines I have, I'm probably most partial to
this machine. I love it's small, robust case.

If possible, use a NiMH specific charger instead of the original HP intended for NiCd. While size and voltage are the same, the chemistry is different and it is advisable to apply the proper charge cycle (current and time) to each type. You can find interesting postings (I think they were from Gene) about this issue in the Museum Archives.

I don't use (or own) any purely financial calcs. My favorite for all-around use is my 41CX. The next best is my 16C, which I use while debugging software, tracing TCP packets, etc. And after that is my Pickett N600-ES pocket slide rule, which I carry everywhere. :-)

In my case, "blown" means with a CMT massive 32RAM in the Card Reader port, the HP 41 Translator Pac (a must), the HP 71 Math Rom, The 3rd party-made HP 71 AMPI Statistics Rom,
the HP 71 Finance Pac, and of course the IL Module. The environmental overlay is there as well to protect the keyboard from spills, or even dirty air molecules.

Add to this 2 41CX's, One Double Speed 41CV (all three used to periodcally "feed" 41 programs to the 71 via the IL loop) the digital cassette drive, the IL printer and God knows how many user written, 41, BASIC, and FORTH programs.

The unit is pristine,with the leather carrying case and is "polished" with Armor All every month, during the monthly battery change. Yes, I do change the Duracells every month. Yes, I am a true 71B fanatic, have been since I bought it when I was a caclulator salesman at UC Berkeley's bookstore from '84-'86.

Gentelemen, in my case, "blown" means "BLOWN" - start your engines ;)

This has been my day-to-day machine since 1984.I'm having a lot of trouble moving over to the 48GX, although I have been trying!

Well, I'm happy to oblige, though actually, I prefer the leather wrapped three-spoke Momo wheels, but that's just me.

For some reason, I just never really warmed to either the 75 or the 71. I don't know why, but they just never apealed to me the way the 41 or the Voyagers did. I guess that with calculators -- as with steering wheels -- it's simply a matter of taste. :^)

I agree on the 75- that was NOT a useful macchine IMHO. Hard to program, no real interfacability and no HP 41 translator capabilities.

The real thing about the 71 was the 41 program running ability and hp 41 connectivity- at about 5X the speed of the 41. You had that massive library of cheap/free programs out there and almost any 71 user probably was a 41 user before and knew which programs he wanted. All he had to do was shoot them over to the 71 (EASY via IL) and run them a lot faster and with a zillion times more onboard storage. They used to call the 71 a "Turbo 41" at Educalc.

Oh, don't get me wrong -- the 71 was a fine machine -- it's just that it never evoked in me the same desire that the 41 and the 1{1|5|6}C did. It could also have something to do with the fact that, at the time, I couldn't really afford one. :^)